Enhanced PDCCH (ePDCCH) is a Rel-11 feature currently standardized in the 3GPP in the Enhanced downlink control channel(s) for LTE Work Item. The main motivation behind ePDCCH is to improve control channel performance (especially capacity and coverage) in the case of new Rel-10/11 features such as CoMP (Coordinated Multipoint Transmission/Reception), DL MIMO (Downlink Multiple Input Multiple Output), Heterogeneous Network and Carrier Aggregation (including extension carrier). In the case ePDCCH is used to schedule downlink data on PDSCH (this is the main use case for ePDCCH), there is a need to provide uplink control channel resource(s) for HARQ-ACK transmitted on PUCCH as response to received PDSCH. The topic has received attention in the last couple of 3GPP RAN WG1 meetings #69 and #69bis.
The latest agreement of these meetings is that the PUCCH Format 1a/1b resource for HARQ-ACK transmission in response to ePDCCH-scheduled PDSCH is at least partly implicitly determined. Which resource index could be used therefore still has to be further discussed. Further, it has been agreed that specification support for avoiding collisions of PUCCH format 1a/1b resources corresponding to ePDCCH and PDCCH is provided. However, it still has to be discussed how to provide such a collision avoidance.
In LTE Rel-8 the implicit PUCCH resource determination is based on the lowest index of the scheduling CCE. With ePDCCH, the CCE concept is replaced with so called eCCEs (enhanced Control Channel Elements). The present invention presents an indexing scheme for ePDCCH eCCEs necessary for supporting implicit resource allocation for HARQ-ACK.
The PUCCH resource allocation for dynamically scheduled PDSCH ACK/NACKs in LTE Release 8 and beyond is based on implicit mapping. To be specific, the index of the lowest PDCCH Control Channel Element (CCE) scheduling the PDSCH determines directly the index of the PUCCH resource used for ACK/NACK signalling, i.e. there is a one-to-one mapping between the CCE and the PUCCH resource indices. An example of this basic principle is illustrated in FIG. 1.
The implicit resource allocation has the benefit that it minimizes the UL overhead as dedicated PUCCH resources do not need to be reserved for each UE. The basic principle is very attractive with ePDCCH as well. As one of the main motivations for the introduction of ePDCCH is increased control channel capacity, the number of scheduled users is expected to increase making efficient PUCCH usage even more important.
As mentioned above, the latest agreement in the 3GPP confirms that DL HARQ-ACK resource allocation on PUCCH will be done at least partly implicitly also in the case of ePDCCH.
Regarding the structure of the ePDCCH, there has been an agreement that at least for the case localized allocations, there are typically 4 (potentially 2 or 3 in some specific cases) eCCEs on a PRB. This results in having potentially a very large number of possible eCCE locations and, assuming one-to-one mapping between an index of the eCCE and the index of the PUCCH resource, in an excessively large UL signalling overhead. Therefore, it seems to be clear that there is a need to develop a more efficient eCCE indexing scheme.
As mentioned above, the implicit ACK/NACK resource allocation in LTE Release 8 is based on the CCE indexing. However, such indexing scheme does not exist currently for ePDCCH.
In LTE Rel-8, the PDCCH resources are divided into CCEs which are then simply numbered. However, such an approach is not an attractive one in the case of ePDCCH as due to the nature of ePDCCH the number of resources and hence also the UL overhead would easily become excessive. This is shown in FIG. 2 which illustrates a straightforward extension of Rel-8 approach to support ePDCCH.